In emission control of a diesel engine, a reduction in PM is as important as a reduction in NOx. As a technique effective for the reduction, DPF is well known.
The DPF is a PM collection device which uses a filter. In an engine operation state where an exhaust gas temperature is low, the PM is continuously accumulated in the DPF so that forced regeneration in which a temperature is forcibly increased and the PM is thereby burnt is performed.
In the forced regeneration of the DPF, late post injection (an injection timing is retarded and combustion is not caused in a cylinder) in which the PM is injected into a cylinder is performed, oxidation reaction is caused in a diesel oxidation catalyst (hereinafter abbreviated as DOC) disposed at a stage prior to the DPF, the temperature in the part of the DPF is increased to a high temperature by heat of reaction, and the PM accumulated in the DPF is thereby burnt.
Consequently, the temperature needs to be increased to the high temperature and, in terms of reducing a forced regeneration time period of the DPF, the temperature of gas passing through the DPF needs to be maintained as high as possible. However, when the temperature of the exhaust gas passing through the DPF is increased to the high temperature in a state where a large amount of the PM is accumulated in the DPF, there is danger that a large amount of the PM is burnt at once and the temperature is excessively increased.
On the other hand, when the gas temperature is set to a low value, the regeneration time period is prolonged and danger that late post injection fuel is dropped into an oil pan from the wall surface in the cylinder and an oil dilution quantity is increased is enhanced.
Accordingly, various improvements and proposals such as control in which an inlet temperature of the DPF is constantly maintained at a target inlet temperature and control in which the target inlet temperature is changed in accordance with the regeneration state of the DPF have been made.
For example, Japanese Patent Application Laid-open No. 2007-239740(Patent Document 1) discloses that an inlet temperature target value of the DPF is determined based on any of a soot accumulation quantity, a soot accumulation quantity change rate, a DPF temperature, and a DPF temperature change rate. regeneration state of the DPF have been made.
In addition, Japanese Patent No. 3951619 (Patent Document 2) discloses a technique in which a target DPF inlet temperature is stepwise changed such that the target temperature is increased to the target temperature at the next step when the target DPF inlet temperature is maintained for a predetermined period of time or longer.
Further, Japanese Patent Application Laid-open No. 2009-138702 (Patent Document 3) discloses that a period of time elapsed since start of the forced regeneration of the DPF is measured, a DPF inlet temperature target value is set to a lower value as the measured period of time is shorter, and a forced regeneration unit sets the injection quantity of sub fuel injection in accordance with the target temperature to perform the sub fuel injection.
Patent Document 1: Japanese Patent Application Laid-open No. 2007-239740
Patent Document 2: Japanese Patent No. 3951619
Patent Document 3: Japanese Patent Application Laid-open No. 2009-138702
However, when the target temperature is set in accordance with the regeneration elapsed time period, there are cases where the actual DPF temperature is completely different from the target temperature due to a difference in operation condition, and hence it is difficult to perform stable control. In addition, in the method using the PM accumulation quantity, it is necessary to estimate the PM accumulation quantity so that the method greatly depends on accuracy in estimation, and hence there is a problem that control logic becomes complicated.